When a sewing machine operator guides two plies of material through the reciprocating needle of a sewing machine during the sewing function, the feed dogs of the sewing machine oscillate continually to engage and pull the lower ply of material through the needle while the presser foot of the machine urges the upper ply of material down toward the feed dogs. The presser foot applies drag to the top ply of material while the feed dogs positively urge the lower ply of material through the sewing machine, and this results in the lower ply of material being moved through the sewing machine at a slightly faster rate than the upper ply of material. When two long pieces of material of equal length are matched together so that their leading and trailing ends are in alignment and pieces are sewn together, the difference in movement between the lower ply and the upper ply of material during the sewing process usually causes the trailing ends of the plies of material to be mismatched at the completion of the sewing operation.
Sewing machine operators that are accustomed to sewing long lengths of material usually hold the plies of material between their fingers, with the upper ply of material being held between the index and middle fingers and the lower ply of material being held between the middle and ring fingers. The fingers of the operator not only guide the layers of material to the machine but also apply a small amount of tension to the portions of the layers of material extending from the fingers of the operator's hand to the feed dogs and presser foot of the sewing machine. The ring and middle fingers usually apply more drag friction to the lower ply of material so that the increased tension in the lower ply of material restrains and stretches the lower ply slightly more than the upper ply, which compensates for the drag friction applied to the upper ply of material by the presser foot of the sewing machine. With this technique, an experienced sewing machine operator frequently is able to match the trailing ends of long pattern parts at the end of a long sewing run.
When a long sewing run is required, as when an inseam or a side seam of trousers is being sewn, the sewing machine and its operator usually must sew over multiple layers of materials that may occur at seams or pockets, etc. The fingers of the sewing machine operator must accommodate the additional plies of material as they are processed toward the sewing machine, and any curves in the matched pattern parts require the operator to turn the plies of material as the plies of material are being fed to the machine. Thus, the procedure to form a commercially acceptable seam in a long length of matched plies of material is sometimes complicated and requires the skill of an experienced and an adept machine operator.